1. Field of the Invention
The present invention relates to a finder optical system, and more particularly, to a real-image type finder optical system employed for a lens shutter camera, etc.
2. Description of the Prior Art
FIG. 1 shows a conventionally known real-image type finder optical system. According to the finder optical system, first, a primary image 30 is formed in the vicinity of a positive first condenser lens 20 by an object lens 100 consisting of a positive and a negative lens elements. A secondary image 60 is formed in the vicinity of a positive second condenser lens 50 by a positive relay lens 45 arranged behind the primary image 30. The relay lens 45 erects the primary image 30 which is inverted upside down and leftside right to form an erecting image. The secondary image 60 is enlarged by a positive eyepiece 70 and viewed by a pupil 80.
A relay part where the relay lens 45 is arranged occupies a large part of the finder optical system. The length of the relay part, that is, a distance L between the primary image 30 and the secondary image 60 is shortest when the relay magnification is 1.times., and a relation of L=4f.sub.R (where f.sub.R represents the focal length of the relay lens 45 is satisfied. Therefore, if the focal length f.sub.R of the relay lens 45 is reduced, the length L of the relay part is decreased, so that the finder optical system becomes compact.
However, in order to reduce the focal length f.sub.R of a conventional lens having curved surfaces as shown in FIG. 1, the radius of curvature of the lens has to be decreased. Since the variation in aberration increases when the radius of curvature is small, it is usually considered that the limit for the focal length f.sub.R to be reduced to is approximately 15 mm. In other words, approximately 60 mm to 100 mm is required for the length L of the relay part. Therefore, it is difficult to reduce the length L of the relay part while maintaining a required optical performance, and consequently, it is inevitable that the length of the finder optical system increases.
On the other hand, as an optical systems which inverts an image upside down and leftside right, an optical system is also known where a reflecting surface (reflecting even times at the up and down sides and the left and right sides, respectively) such as a Porro prism, etc. is employed in place of the above-described relay lens. However, the Porro prism requires a large space for providing the reflecting surface, etc., it is inevitable that the length of the finder optical system increases, similarly to the case where the relay lens is employed.